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Energy absorbers used against impact loading
Nuclear Engineering and Design 37 (1976) 407-412 © North-Holland Publishing Company
ENERGY ABSORBERS USED AGAINST IMPACT LOADING * T. KUKKOLA Nuclear Power Project Group, Imatran Voima Osakeyhtio', HelsinkL Finland Received 18 December 1975 In the WWER-440 reactor the primary piping consists of six horizontal loops going rapidally from the pressure vessel, each loop having a horizontal steam generator. In this reactor type the relatively long primary piping with many curved sections requires special attention in order to successfully eliminate the consequences of the design basis accident. Emergency supports are located in appropriate places to restrict the movements of the pipe. Under normal conditions there is a gap of some centimeters between the pipe and a support so that in the pipe can be deformed freely under changing loads. This paper deals with those energy-absorbing structures used at the Loviisa Nuclear Power Plant for protection against impact loading. Places and circumstances where energy-absorbing structures are employed are specified. Development and design of impact absorber elements are discussed and impact tests are described.
1. Introduction
(C) Between the emergency restraints and pipes in other circuits; for example pressurizing system and emergency core cooling system.
This paper deals in closer detail with those energyabsorbing structures used at the Loviisa Nuclear Power Plant for protection against impact loading. Such loading may be caused, among others, by pipe break accidents or spent fuel cask drop accidents. This paper is also related to the discussions at the Third SMiRT Conference [1].
2. Places and circumstances where energy-absorbing
structures are employed (a) Between primary circuit pipes and emergent restraints. Fig. 1 shows the piping principle and the emergency restraints for one steam generator. Pipe size 560 × 32 mm, operating pressure 125 bar, temperature 300°C. (b) Between secondary circuit pipes and emergency restraints. Fig. 2 shows the piping principle and the emergency restraints. The size of the steam pipe is 465 × 16 mm and that of the feed water pipe ¢ 273 × ! 5 mm. Steam pressure 44 bar, temperature 255°C.
* Paper Q3/4 presented at International Seminar on Extreme Load Conditions and Limit Analysis Procedures for Structures (ELCALAP), Berlin, Germany, 8-11 September 1975.
Fig. l. Principle of primary piping and restraining at the Loviisa NPS.
407
408
T. Kukkola /Energy absorbers used against impact loading '# 60,~ x 3 2 t ;//ed
mm
w,'lh Leca- ~On~rete ~, t.f5
Lood,'n~l / H~'
d,,¢
• 60.3 x3,2 ~
r{//ed wifh
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/
60
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w/fh Leca f.l.0~k_~_JI
Jlr'/
Leca -to~(r~le T = ,g? kc~/d~'~J
u./,'//~d p,'ee 7-
co~c: ret'e
~0
30
/
20
10
(a) o
fO
EO
30
qO
5O
60
Oe~ormahon
~ram
Fig. 2. Principle of secondary piping and restraining at the Loviisa NPS. (d) In the restrainers of the steam generator displacement in accident conditions. (e) At those spots of the conveying passage of the spent fuel cask where a cask drop may cause a rupture of the containment. Between the piping and the emergency restraint there is a space of 5 0 - 2 0 0 mm for the impact absorbers, and the maximum thermal and pressure movement is 85 ram. The maximum free drop height of the spent fuel cask in these rooms is approx. 15 m.
3. Developing the impact absorbers The first experiments were made with round steel pipes, pressing each of them together laterally. Fig. 3 (a) shows experimental results [2]. Part of the pipes were filled with LECA concrete, with densities ranging between 0.97 and 1.15 kg/dm 3. Fig. 3(b) shows a pressed pipe after the experiment. The averaging level of the pressing force was too low for the pipe to absorb sufficient energy• Experiments were continued with rectangular steel pipes, again pressed together laterally. Fig. 4(a) shows
(b) Fig. 3. (a) Loading deformation curves of filled and unfilled
pipes when loaded laterally. (b) Filled pipe after loading experiment. experimental results. The ends of some pipes were closed with welded steel plates. Fig. 4(b) shows the pressed pipe specimens. In these experiments the level of the pressing force was too low for 100% energy absorption. On the other hand, there was too little room for joining several pipes to function simultaneously as impact absorbers.
4. Practicable structural solution for impact absorber elements Good results were obtained by loading in the direction of the axis of the rectangular pipes. Figs. 5 and 6
T. Kukkola I Energy absorbers used against impact loading F/He
~E~
~0~ Z O o
the lateral length of the quadrangle, the level of the pressing force curve is horizontal. The curves on the right (figs, 5 and 6) show that several elements can be joined in a row by welding them together with intermediate plates. Also in this case, the level of the pressing force curve will be horizontal.
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(b), Fig. 4. (a) Force-displacement curves of laterally loaded rectangular pipes. (b) Two rectangular pipe specimens after experiment.
show pressing curves of pipe elements of various sizes. The curves on the left (figs. 5 and 6) show the dependence of the pressing curve on the height of the element. If the element height is approximately half of
(b)
410
T. Kukkola / Energy absorbers used against impact loading
(c) Fig. 5. (a) Force-displacement corves of different sizes, heights and number of elements. (b) Experimental specimen after loading. (c) Experiment in progress. Fig. 5(b) shows pressed experiment specimens after loading. When the average pressing force was divided by the cross-sectional metal area of the rectangular
5. Absorber dimensioning principles in connection with
o.go
hO
pipe an average value of 55 k p / m m 2 was obtained (fig. 5(c)). The material was steel St 42-2. When the pipe elements are welded to intermediate plates, the construction allows loading in directions other than that of the length axis. Fig. 7(a) shows experimental results when arrangements were as shown by fig. 7(b). Fig. 7(c) shows impact absorbing elements after the experiment. The experiments indicate that several pipe elements can be joined in a row. The maximum number of elements which can be joined together in a row is set by the elastic buckling length of the whole structure, if buckling sideways is not restricted. Fig. 8(b) shows an absorber structure where the ten dency to buckle sideways has been restricted, whereas fig. 8(a) gives the force-pressing curve of the same absorber. This absorber consists of eight parallel rows of 19 elements each. The absorber is a small-scale model of the absorber which will be employed to protect structures against drop accidents of spent fuel casks. The pressing force of the actual absorber will be 1600 Mp and the pressing distance 70 cm. Fig. 9 shows some examples of practical absorber solutions.
/ /
emergency restraints of piping
40~4o~2.¢J
30
Roughly, the following procedure is applied: (a) the jet force on the pipe is defined; (b) the free clearance in the movement direction of the pipe is defined; (c) the space available for the absorber is verified which gives the pressing distance of the absorber; (d) the level of the absorber pressing force is chosen so that the deformation work capacity of the absorber is at least as great as the work done by the force on the pipe; and (e) the emergency restraint is dimensioned so that it can bear the pressing force of the absorber.
20
20
fO
30
F/MH
~0
fro
LlO.qO
F~ M 2 fro
200
~F
150
fSO
4~ 100,
,°°
F
6. Impact experiments
f 50
~0
20
~
60
$0
20
~0
60
80 ~a
Fig. 6. Force-displacement curves of different sizes, height and number of elements.
The experiments were static, and presses with 60 and 300 Mp capacities were used at +20°C temperature. The absorbers were fixed onto the pipes over an asbestos fabric with the operating temperature of max. +150°C. The velocity of the impact to the emergency
lq0.
I F
80.
F
@ 20
•
20
'I0
(,0
80
~00
,~/,,,m
(a)
~20
(b)
(c) Fig. 7. (a) Force-displacement curves with different loading directions. (b) Arrangement of experiments giving the curves of fig. 7(a). (c) Impact absorbing elements after loading experiment.
412
T. K u k k o l a / Energy absorbers used against impact loading
F//~p 300t
A
;~ ~h~ real ~frudure
_~T6oo ~
a00-
100.
E I
(a)
50
fO0
150
'
200
,~50
_~ d l m m
Fig. 9. Some existing constructions for primary circuit and steam pipes.
restraint is calculated at approx. 5m/sec in a primary pipe break accident. The former decreases the yielding of the material, the latter increases it. An absorber of the type shown in fig. 8(a) is to be tested shortly by striking it together with a pile driver used for foundation work at construction sites. This will provide information about the behaviour of the absorber in real conditions.
References
(b) Fig. 8. (a) Force-pressing curve of impact absorber shown in fig. 8(b). (b) Small-scale model of absorber against drop accidents as opened up after experiment.
[ 1 ] K. Ikonen, H. Reijonen and V.M. Kangas, Structural analysis of piping after a large pipe break in a WWER-440 type reactor, Paper F5/2, Transactions of the 3rd International Conference on Structural Mechanics in Reactor Technology, London, Sept. (1975) Vol. 2, Part F. [2] Research Reports by the Concrete and Soil Laboratory of Imatran Voima OsakeyhtiS.
ENERGY ABSORBERS USED AGAINST IMPACT LOADING * T. KUKKOLA Nuclear Power Project Group, Imatran Voima Osakeyhtio', HelsinkL Finland Received 18 December 1975 In the WWER-440 reactor the primary piping consists of six horizontal loops going rapidally from the pressure vessel, each loop having a horizontal steam generator. In this reactor type the relatively long primary piping with many curved sections requires special attention in order to successfully eliminate the consequences of the design basis accident. Emergency supports are located in appropriate places to restrict the movements of the pipe. Under normal conditions there is a gap of some centimeters between the pipe and a support so that in the pipe can be deformed freely under changing loads. This paper deals with those energy-absorbing structures used at the Loviisa Nuclear Power Plant for protection against impact loading. Places and circumstances where energy-absorbing structures are employed are specified. Development and design of impact absorber elements are discussed and impact tests are described.
1. Introduction
(C) Between the emergency restraints and pipes in other circuits; for example pressurizing system and emergency core cooling system.
This paper deals in closer detail with those energyabsorbing structures used at the Loviisa Nuclear Power Plant for protection against impact loading. Such loading may be caused, among others, by pipe break accidents or spent fuel cask drop accidents. This paper is also related to the discussions at the Third SMiRT Conference [1].
2. Places and circumstances where energy-absorbing
structures are employed (a) Between primary circuit pipes and emergent restraints. Fig. 1 shows the piping principle and the emergency restraints for one steam generator. Pipe size 560 × 32 mm, operating pressure 125 bar, temperature 300°C. (b) Between secondary circuit pipes and emergency restraints. Fig. 2 shows the piping principle and the emergency restraints. The size of the steam pipe is 465 × 16 mm and that of the feed water pipe ¢ 273 × ! 5 mm. Steam pressure 44 bar, temperature 255°C.
* Paper Q3/4 presented at International Seminar on Extreme Load Conditions and Limit Analysis Procedures for Structures (ELCALAP), Berlin, Germany, 8-11 September 1975.
Fig. l. Principle of primary piping and restraining at the Loviisa NPS.
407
408
T. Kukkola /Energy absorbers used against impact loading '# 60,~ x 3 2 t ;//ed
mm
w,'lh Leca- ~On~rete ~, t.f5
Lood,'n~l / H~'
d,,¢
• 60.3 x3,2 ~
r{//ed wifh
B~
/
60
..o.,,,.,
f;//ed
w/fh Leca f.l.0~k_~_JI
Jlr'/
Leca -to~(r~le T = ,g? kc~/d~'~J
u./,'//~d p,'ee 7-
co~c: ret'e
~0
30
/
20
10
(a) o
fO
EO
30
qO
5O
60
Oe~ormahon
~ram
Fig. 2. Principle of secondary piping and restraining at the Loviisa NPS. (d) In the restrainers of the steam generator displacement in accident conditions. (e) At those spots of the conveying passage of the spent fuel cask where a cask drop may cause a rupture of the containment. Between the piping and the emergency restraint there is a space of 5 0 - 2 0 0 mm for the impact absorbers, and the maximum thermal and pressure movement is 85 ram. The maximum free drop height of the spent fuel cask in these rooms is approx. 15 m.
3. Developing the impact absorbers The first experiments were made with round steel pipes, pressing each of them together laterally. Fig. 3 (a) shows experimental results [2]. Part of the pipes were filled with LECA concrete, with densities ranging between 0.97 and 1.15 kg/dm 3. Fig. 3(b) shows a pressed pipe after the experiment. The averaging level of the pressing force was too low for the pipe to absorb sufficient energy• Experiments were continued with rectangular steel pipes, again pressed together laterally. Fig. 4(a) shows
(b) Fig. 3. (a) Loading deformation curves of filled and unfilled
pipes when loaded laterally. (b) Filled pipe after loading experiment. experimental results. The ends of some pipes were closed with welded steel plates. Fig. 4(b) shows the pressed pipe specimens. In these experiments the level of the pressing force was too low for 100% energy absorption. On the other hand, there was too little room for joining several pipes to function simultaneously as impact absorbers.
4. Practicable structural solution for impact absorber elements Good results were obtained by loading in the direction of the axis of the rectangular pipes. Figs. 5 and 6
T. Kukkola I Energy absorbers used against impact loading F/He
~E~
~0~ Z O o
the lateral length of the quadrangle, the level of the pressing force curve is horizontal. The curves on the right (figs, 5 and 6) show that several elements can be joined in a row by welding them together with intermediate plates. Also in this case, the level of the pressing force curve will be horizontal.
SO,~iO0 - h.5"
~
409
~./~--~~ ."
F÷ F/PIp ~£~.~.e~ F/H, e0
'~
~
30+$0+30
H, ~0+~0
80
fO0~ . ~ f ~0 2b
H:~II
100
qO
~b
~
88,saa'R,~ti.i
!
f$'O
/
~. ,
20
Vlrl, .o
~0
,,.s,.s,// ~/
L
~......
//ho.,°.,° /
qO0
(a)
-
;o
~'o Ill
~o ~
~
so ~
-
,$/,-m
,oo
I
10
'r
(a)
11
(b), Fig. 4. (a) Force-displacement curves of laterally loaded rectangular pipes. (b) Two rectangular pipe specimens after experiment.
show pressing curves of pipe elements of various sizes. The curves on the left (figs. 5 and 6) show the dependence of the pressing curve on the height of the element. If the element height is approximately half of
(b)
410
T. Kukkola / Energy absorbers used against impact loading
(c) Fig. 5. (a) Force-displacement corves of different sizes, heights and number of elements. (b) Experimental specimen after loading. (c) Experiment in progress. Fig. 5(b) shows pressed experiment specimens after loading. When the average pressing force was divided by the cross-sectional metal area of the rectangular
5. Absorber dimensioning principles in connection with
o.go
hO
pipe an average value of 55 k p / m m 2 was obtained (fig. 5(c)). The material was steel St 42-2. When the pipe elements are welded to intermediate plates, the construction allows loading in directions other than that of the length axis. Fig. 7(a) shows experimental results when arrangements were as shown by fig. 7(b). Fig. 7(c) shows impact absorbing elements after the experiment. The experiments indicate that several pipe elements can be joined in a row. The maximum number of elements which can be joined together in a row is set by the elastic buckling length of the whole structure, if buckling sideways is not restricted. Fig. 8(b) shows an absorber structure where the ten dency to buckle sideways has been restricted, whereas fig. 8(a) gives the force-pressing curve of the same absorber. This absorber consists of eight parallel rows of 19 elements each. The absorber is a small-scale model of the absorber which will be employed to protect structures against drop accidents of spent fuel casks. The pressing force of the actual absorber will be 1600 Mp and the pressing distance 70 cm. Fig. 9 shows some examples of practical absorber solutions.
/ /
emergency restraints of piping
40~4o~2.¢J
30
Roughly, the following procedure is applied: (a) the jet force on the pipe is defined; (b) the free clearance in the movement direction of the pipe is defined; (c) the space available for the absorber is verified which gives the pressing distance of the absorber; (d) the level of the absorber pressing force is chosen so that the deformation work capacity of the absorber is at least as great as the work done by the force on the pipe; and (e) the emergency restraint is dimensioned so that it can bear the pressing force of the absorber.
20
20
fO
30
F/MH
~0
fro
LlO.qO
F~ M 2 fro
200
~F
150
fSO
4~ 100,
,°°
F
6. Impact experiments
f 50
~0
20
~
60
$0
20
~0
60
80 ~a
Fig. 6. Force-displacement curves of different sizes, height and number of elements.
The experiments were static, and presses with 60 and 300 Mp capacities were used at +20°C temperature. The absorbers were fixed onto the pipes over an asbestos fabric with the operating temperature of max. +150°C. The velocity of the impact to the emergency
lq0.
I F
80.
F
@ 20
•
20
'I0
(,0
80
~00
,~/,,,m
(a)
~20
(b)
(c) Fig. 7. (a) Force-displacement curves with different loading directions. (b) Arrangement of experiments giving the curves of fig. 7(a). (c) Impact absorbing elements after loading experiment.
412
T. K u k k o l a / Energy absorbers used against impact loading
F//~p 300t
A
;~ ~h~ real ~frudure
_~T6oo ~
a00-
100.
E I
(a)
50
fO0
150
'
200
,~50
_~ d l m m
Fig. 9. Some existing constructions for primary circuit and steam pipes.
restraint is calculated at approx. 5m/sec in a primary pipe break accident. The former decreases the yielding of the material, the latter increases it. An absorber of the type shown in fig. 8(a) is to be tested shortly by striking it together with a pile driver used for foundation work at construction sites. This will provide information about the behaviour of the absorber in real conditions.
References
(b) Fig. 8. (a) Force-pressing curve of impact absorber shown in fig. 8(b). (b) Small-scale model of absorber against drop accidents as opened up after experiment.
[ 1 ] K. Ikonen, H. Reijonen and V.M. Kangas, Structural analysis of piping after a large pipe break in a WWER-440 type reactor, Paper F5/2, Transactions of the 3rd International Conference on Structural Mechanics in Reactor Technology, London, Sept. (1975) Vol. 2, Part F. [2] Research Reports by the Concrete and Soil Laboratory of Imatran Voima OsakeyhtiS.